Barrier height factors which increase

For the DTO model we must have an estimate of the torsional vibration frequency and the barrier to internal rotation of the constituent monomers. The DTO model fits the experimental data for bulk polymer if H = 5.4 kcal/mole, vt — 1012 c.p.s., and Zt = 30 which are not unreasonable values. One would expect the barrier height to decrease upon dilution (if it changes at all) as the chain environment loosens up. Assuming that rotation about C—O—C bonds is predominate, we take the experimental values of H = 2.63 kcal/mole, vt = 7.26 x 1012 c.p.s. of Fateley and Miller (14) for dimethyl ether. Eq. (2.8) predicts rSJ° = 0.47 X 10-8 sec at 253° K with Zt = 30. We shall use this as our dilute solution result. [The methyl pendant in polypropylene) oxide will act to increase the barrier height due to steric effects, making this calculated relaxation time somewhat low for this choice of a monomer analog.] Tmax is seen to change only by a factor of 102—103 upon dilution in the DTO model. 

Typical room temperature current-voltage (Z-V) characteristics of Ni/Au SDs are plotted in Figure 6.13. As we can see, the saturation current decreases monotonously with increasing SiN.r deposition time from 0 (the control sample) to 5 min which means that the effective Schottky barrier height increased owing to shallow defect reduction. Meanwhile, the series resistance and ideality factor also decreased when longer SiN deposition times were used. Based on the thermionic emission model, the forward current density at V > 3kT/q has the form [11] ... 

SiN nanonetwork, the barrier height is 0.76 eV. When the SiN deposition time is increased, the barrier height increases from 0.84 (3 min SiN ) to 1.13 eV (5 min SiN (). At the same time, the ideality factor reduces from 1.3 (no SiN ) to 1.06 (5 min SiN ) which indicates that the SDs are nearly ideal in samples grown with the SiN nanonetwork. Incidentally, this improved value is consistent with the work function of Ni (5.2 eV) and the electron affinity of GaN (4.1 eV). In the literature, a value of 1.099 eV (Ni) barrier height was achieved only after the GaN surface was treated with (NH S [12], which is known to passivate the surface defects albeit temporarily. Our results indicate that the Ni Schottky barrier height is very sensitive to the crystalline quality and the excess current... 

The same structural analysis techniques have been applied to study several other metals, including Cr, Ni, and Pt (Nemanich e( al, 1983) that are known to form silicides, and Al and Au (Tsai et al, 1982, 1983), which do not. For the metals that form silicides, it was found that atomic interdiflFu-sion, which resulted in a disordered phase, often preceded silicide formation. Then, silicide formation occurred at an elevated temperature. The results are summarized in Table I. For all these cases the internal photoemission showed no change in barrier height (<0.03 eV) after annealing, but the photoemission signal increased by a factor of two. For all the metals that form silicides it appears that the reactions occur at temperatures similar to those on crystalline Si. The disordered intermixed phase, however, has not been definitively observed for metals on crystalline Si. 

The availability of three isotopes of hydrogen allows the comparison between the behaviour of any two of these experimentally and the comparison to that calculated by absolute reaction rate theory. Since the isotope effect cancels out the major uncertainty in absolute reaction rate theory in calculations of rate, namely the barrier height, the theory puts some sharp restrictions on the magnitude of the isotope effect and its temperature dependence [10]. However, the semi-classically calculated isotope effect k"//cP is multiplied by the factor a number greater than unity which increases as the... 

The height of the potential barrier decreases with the decrease of the transfer distance. Therefore, the contribution of the transitions between excited vibrational states increases and so does the transition probability. However, short-range repulsion between the reactants increases with a decrease of R, and the reaction occurs at an optimum distance R which is determined by the competition of these two factors. In principle, we may imagine the situation when the optimum distance R corresponds to the absence of a potential barrier for the proton. However, we should keep in mind that the transitions between certain excited states may become entirely adiabatic at short distances.40,41 In this case, the further increase of the transition probability with the decrease of R becomes quite weak, and it cannot compensate for the increased repulsion between the reactants, so that even for the adiabatic transition, the optimum distance R may correspond to sub-barrier proton transfer. 

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